To adopt a photoreceptor as a photodiode of a receiving circuit for free-space optical communication, the receiving circuit receiving and recognizing a faint optical signal which is supplied from a sending circuit and propagated through a space, it is important to prevent the malfunction caused by shot noise.
Provided that a current i flows in the photodiode, shot noise at a PN junction of the photodiode is represented as √(2×q×i×Δf). In other words, the shot noise increases in proportion to the power root of a current value. In the foregoing expression, “q” is an elementary charge and “Δf” is a frequency band.
When a signal is received in complete darkness, i.e. when a signal is received in a condition with no direct component of light, no current flows in the photodiode so that there is no shot noise at the PN junction of the photodiode.
However, if the threshold voltage of a signal detection circuit is set so as to optimize the sensitivity for a condition that a signal is received with no shot noise (cf. left figure in FIG. 4), a direct current flows into the photodiode in a bright condition (i.e. with a direct component of light), so that the shot noise is generated. This shot noise is amplified along with the signal, and supplied to the signal detection circuit. As a result, the signal detection circuit detects both the signal and the noise, thereby malfunctioning (cf. right figure in FIG. 4).
To prevent this malfunction, it is necessary to set the threshold voltage of the signal detection circuit in such a manner as to deteriorate the sensitivity in darkness. Setting the threshold voltage in this way, however, deteriorates the overall sensitivity.
To prevent such deterioration of the sensitivity, a circuit shown in FIG. 2 has been proposed. In FIG. 2, a current generated in a photodiode PD turns to a corrector current of an NPN transistor Q1 which constitutes, together with a resistor R3, a grounded-base circuit in a current-voltage conversion circuit 11, and is converted to a voltage by a current-voltage conversion resistor R1. Subsequently, only an AC component is drawn by an AC coupled circuit (differentiation circuit) made up of a capacity C1 and a resistors R4 and R5, and then amplified by an amplifier circuit AMP1. Subsequently, an AC component is further drawn by an AC coupled circuit (differentiation circuit) made up of a capacity C2 and a resistor R6. The AC component having been drawn is, by a comparator COMP, compared with a threshold voltage of a threshold voltage generating circuit SVG, so that an optical signal superposed on the incoming light is detected.
When such a conventional circuit is used in a condition with a direct component of light, a DC current flows into the photodiode PD, causing the voltage drop in the resistor R1. As the voltage drop exceeds a forward voltage in a diode D1, a current generated by the photodiode PD is divided so as to reach the resistors R1 and R2. With this, a current voltage conversion resistor value is reduced to be substantially identical with a parallel resistance of the resistors R1 and R2, and hence the conversion gain decreases. In this manner, a noise voltage caused by the shot noise generated in the photodiode PD is reduced, and such malfunction that a noise is mistakenly recognized as a signal is prevented.
However, the conventional circuit shown in FIG. 2 has the following drawback.
The collector voltage of the transistor Q1 varies in accordance with a direct current flowing in the photodiode PD, and this direct current varies in accordance with incoming light. For this reason, when the collector voltage of the transistor Q1 is direct-coupled with the amplifier circuit AMP1, a DC component of the collector voltage of the transistor Q1 is multiplied by the gain. The gain of the amplifier circuit AMP1 is generally set so as to be about 200 times as much as the DC component, in order to increase the sensitivity to detect a signal. Thus, when the DC component of the collector voltage of the transistor Q1 varies for 0.7V, the variation of the output of the amplifier circuit AMP1 reaches 0.7×200=140V. Such a large voltage is impractical because the power supply voltage of the amplifier circuit AMP1 has to be higher than this voltage. In practice, the power-supply voltage has to be about a few volts. On this account, it is necessary to cut the DC component of the collector voltage of the transistor Q1 and allow only the AC component to enter the amplifier circuit AMP1. To do so, the AC coupled circuit made up of the capacity C1 and the resistor R4 has to be provided in the previous stage of the amplifier circuit AMP1.
Furthermore, because of the offset by which the amplifier circuit AMP is always accompanied and the relative accuracy of the resistors R4 and R5, the output from the amplifier circuit AMP1 fluctuates. For instance, when an offset on the input of the amplifier circuit AMP1 is 1 mV, an offset on the output from the amplifier circuit AMP1 is 0.2V (i.e. gained for 200 times). If this amplifier circuit AMP1 is directly connected to the comparator COMP of the signal detection circuit 12, the detecting sensitivity fluctuates for the amount of the offset voltage. To prevent this fluctuation, another AC coupled circuit made up of a capacity C2 and a resistor R6 has to be provided between the output of the amplifier circuit AMP1 and the comparator COMP.
In this manner, in the conventional circuit in FIG. 2, two AC coupled circuits have to be provided between the current voltage conversion circuit 11 and the comparator COMP. With these two AC coupled circuits, a signal pulse is differentiated twice as shown in FIG. 3, so that the signal voltage rises at the end of the signal. Thus, it looks as if the signal continues even after the end thereof, and the comparator COMP of the signal detection circuit 12 may mistakenly detect a nonexistent signal.